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Aphasia is a term that describes language dysfunctions resulting from damage to the brain due to various causes like stroke, head injury, tumor or infections. Cerebral cortex is the seat of all language functions, and there are ample clinical evidences associating specific language dysfunctions with discrete brain regions. In fact, awareness on brain language functions were deducted from such observations. There are also evidences for a relation between the handedness (dexterity) of an individual and language lateralization in the brain. More than 90% of the population are right-handed, and among 99% of the right-handed individuals, the left side of the brain (cerebral cortex) is dominant for language functions. Also, the left side of the brain is dominant for the language in ~ 65% of the left-handed individuals. The right hemisphere is dominant for language in about 10-15% of the left-handed individuals and in <1% of the right-handed individuals. The ambidextrous group has bihemispheric language representations.

While the left hemisphere is dominant for language functions, different lobes of the brain act as centres for specific modes of language transaction such as expression, comprehension, repetition, reading, writing, naming and mathematical operations. Accordingly, there are different types of aphasias: Broca’s Aphasia or expressive aphasia, where the verbal expression is predominantly affected, Wernicke’s aphasia or receptive aphasia, where comprehension is predominantly affected, global aphasia where expression as well as comprehension is affected, and Conduction aphasia, where repetition is the dominant function that is affected. Transcortical aphasia are a group of aphasic syndromes where the dysfunctions are due to lesions that separate the language areas from the rest of the brain. The dominant feature of these types of aphasia are that the repetition is intact, and may even go to the extreme, resulting in echolalia or perseveration. Transcortical aphasias are classified in to Transcortical motor, Transcortical sensory or Mixed transcotical, almost parallel to the major classification of aphasia. Some aphasiologists apply the function of repetition for the classification of aphasias into two major groups, repetition retained or lost. While the ability for repetition is lost in Broca’s Aphasia, Wernicke’s aphasia, global aphasia, and conduction aphasia, the function is intact in transcortical aphasia. Nominal aphasia or anomia refers to the inability of individuals to recollect or name an object, person, or place. Anomia is often designated as the hallmark of all aphasias, and also as an early sign of language dysfunction in dementia. The term alexia refers to the inability to read and comprehend, agraphia to the inability to write and acalculia to the inability to perform mathematical operations. Recent advances in neuro imaging has identified the role of several subcortical structures like basal ganglia, thalamus, corona radiata, internal capsule and cerebellum in language processing; lesions in these structures can lead to aphasia. Aphasias resulting from lesions in these structures are, however, atypical in several aspects, and are collectively known as subcortical aphasias. Functional neuro imaging has revealed that such aphasia could also be due to dysfunction at cortical level, because of which they are often considered as disconnection syndromes. The role of right hemisphere in language processing has also been evaluated extensively. It is believed that the right hemisphere sub serves several functions of language including prosody, metaphore analysis and music. Moreover, it has corresponding functions in reading (prosodic reading, reading between lines, comprehension of metaphore), and writing (orthographic, form, size and shape of letters). Right hemisphere lesion can produce aprosody, amusia and dysorthographia.

Current research on aphasia holds the view that while different centres in the right and left hemispheres of the brain specifically control discrete aspects of language functions, they are interconnected through neuronal networks, and function as a parallel computing system in continuum, in an ongoing language task.

There is another condition, primary progressive aphasia, which was earlier confused with true aphasias. It is a rare neurological syndrome that impairs language capabilities; symptoms begin gradually, sometimes before age 65, and tend to worsen over time. Affected individuals may have trouble expressing their thoughts and comprehending or finding words. They may become mute and may eventually lose the ability to understand written or spoken language. They may continue caring for themselves and participating in daily life activities for several years after the onset of the disorder since the condition progresses slowly. This condition is now considered as dementia.

The development of normal language functions can also be delayed, deviant or deficient. However, the term dysphasia is more commonly used rather than aphasia in children with language dysfunctions. Dysphasia in children can be classified into expressive language disorder, receptive language disorder, mixed language disorder, specific speech articulation disorder, acquired aphasia with epilepsy, and developmental disorders of speech and language unspecified. These are situations characterized by language impairment in the absence of hearing loss, emotional disturbances, intellectual deficits, and structural abnormalities (neurological or speech mechanism abnormalities). Aphasia can also occur in children due to discrete damage to the brain that occurred after attaining normal language development. These are called true aphasias, which is similar to adult aphasia caused by stroke, head injury, tumour, seizure disorder, encephalitis or other forms of brain damage. However, such aphasias can be transient because of extreme plasticity of child's brain.

We need to know that language is an inevitable part of life of a human being. Language is essential in interpersonal communication to express one's ideas and to externalize the internal thought process. When a person losses his ability to communicate with others the agony is beyond endurance that it interferes with his/her function as a human being in a society.

Author (Year) Definition
Goodglass and Kaplan(1972) Aphasia refers to the disturbance of any or all of the motor and sensory skills, association and habits of spoken or written language, produced by injury to certain brain areas specialized for these functions.
Darley et al (1975) A multi modality reduction in the capacity to decode and encode meaningful linguistic elements. It is manifested as difficulties in listening, reading, speaking and writing.
Benson(1979) Aphasia is the loss or impairment of language caused by brain damage.
Goodglass et al (1981); Damsio (1981) Aphasia is the disturbance of one or more aspects in the complex process of comprehending and formulating verbal messages that result from newly acquired diseases of the central nervous system.
Martin (1981) Aphasia is a cognitive deficit. “The disorder (Aphasia) is the reduction, because of brain damage, of the efficiency of action and interaction of the cognitive processes that support language behaviour. All the processes, by which sensory information is transformed, reduced, elaborated, stored, recovered, and used.”
Darly (1982) Aphasia is an impairment, as a result of brain damage, of the capacity for interpretation and formulation of language symbols.
McNeil (1982) Aphasia is a multimodality physiological inefficiency with greater than loss of verbal symbolic manipulations. In isolated forms, it is caused by facial damage to cortical and/or subcortical structures of the hemisphere dominant for such symbolic manipulations.

The actual prevalence of aphasia is not available from several parts of the world, especially from the developing countries. The prevalence of aphasia in different countries are given in the following table.

The statistics are calculated by extrapolation of several prevalence or incidence rates against the population of a particular country/region. The statistics used for prevalence/incidence estimation of aphasia are typically based on US, UK, Canadian or Australian data. This extrapolation calculation is automated, and it also considering any genetic, cultural, environmental, social, racial or other differences across various countries/regions for which the extrapolated aphasia statistics below refer to. As such, these extrapolations may be highly inaccurate, especially for the developing or third-world countries, and may give only a general indication (or, even a meaningless indication) as to the actual prevalence or incidence of aphasia in that region.